(a) Field of the invention:
The present invention relates to a light absorptive film having an anti-reflecting property.
(b) Description of the prior art:
As is known to those skilled in the art, the optical system of a phase contrast microscope has such a construction as shown in FIG. 1. Speaking concretely with reference to this drawing, the reference numeral 1 represents a condenser lens, the reference numeral 2 designates an annular aperture stop arranged at the front focal point of said condenser lens or in the vicinity thereof, the reference numeral 3 denotes an object to be subjected to microscopy, the reference numeral 4 represents an objective lens, the reference numeral 5 designates a phase plate arranged at the rear focal point of the objective lens 4 or in the vicinity thereof and at the position conjugate with the annular aperture stop with regard to the condenser lens 1 and objective lens 4, and the reference numeral 6 denotes the position of an image of the object formed with the objective lens 5.
In the phase contrast microscope having the composition as described above, the light emitted from a light source is passed through the aperture 2a of the annular aperture stop, made into parallel rays by the condenser lens 1 and passes through the object. An image of the clear object 3 is formed on an image plane 6 by causing a phase difference of 1/4 wavelength and an adequate absorption of the zero-order diffracted light having passed through the specimen with the phase plate 5.
Further, a high resolution microscope can be composed by arranging an annular filter in place of the phase plate in the optical system shown in FIG. 1 as described in Japanese unexamined published patent application No. 12615/81.
In the phase contrast microscope or the high resolution microscope, flare or ghost may be produced due to the light reflected on the surface of the phase plate or the annular filter used in the microscope. It is therefore an important requirement for the phase plate used in the phase contrast microscope or the annular filter used in the high resolution microscope to eliminate the light reflected on the surface thereof. That is to say, it is possible to prevent flare or ghost from being produced by equipping the phase plate or annular filter with a function for anti-reflection, which is an important factor for governing performance of the phase plate or annular filter. As an example of light absorptive film (phase plates, annular filters, etc.) having such a function for anti-reflection, the one disclosed by Japanese unexamined published patent application No. 8107/81 has already been known to those skilled in the art. This light absorptive film has a composition consisting of two layers, i.e., a metallic layer b and a dielectric layer a arranged on the incidence side thereof, as shown in FIG. 2. In this light absorptive film, let use represent refractive index of the dielectric layer a by n.sub.1 and designate complex refractive index of the metallic layer b by n.sub.2. Let us further assume that a glass plate having a refractive index n.sub.0 is arranged on the incidence side of the dielectric layer a, and a dielectric layer having a refractive index n.sub.3 is arranged on the emerging side of the metallic layer b of the light absorptive film. Moreover, let use represent amplitude reflection factor (Fresnel's reflection coefficient) of the boundary surface 1S between the dielectric layer and upper layer (glass plate) by r.sub.0, designate amplitude reflection factor of the boundary surface 2S between the dielectric layer a and metallic layer b by r.sub.1, and denote amplitude reflection factor of the boundary surface 3S between the metallic layer b and substance (dielectric layer) located on the emerging side thereof by r.sub.2. Then, in order to equip the light absorptive film shown in FIG. 2 with an anti-reflecting function, it is important to cancel the light reflected by the boundary surface 2S having high reflectance by using a light reflected on another surface. For this purpose, it is necessary to select refractive indices of the dielectric layer a and metallic layer b so as to satisfy conditions of n.sub.1 .noteq.n.sub.3 and .vertline.r.sub.2 .vertline.&lt;.vertline.r.sub.2 .vertline. since light is attenuated while passing through the metallic layer b. Further, in order to cancel the light reflected on the boundary surface 3S having such a high reflectance with the light reflected on the boundary surface 2S, it is important to select thickness of the metallic layer b so that the light reflected on the boundary surface 3S and reaching the boundary surface 2S will be in a phase neary opposite to that of the light reflected on the boundary surface 2S (to produce a phase difference of .pi.). The light absorptive film disclosed by Japanese unexamined published patent application No. 12615/81 is designed for anti-reflection as described above.